This invention relates generally to apparatus for logging earth boreholes and specifically to methods and apparatus utilized to assist well logging instruments in traversing highly deviated earth boreholes.
In the practice of obtaining lithological measurements of subsurface formations there is an attendant problem of moving a logging instrument through an earth borehole, and in particular one which is inclined from the vertical and may contain numerous irregularities and obstructions which impede movement of the instrument to such an extent that it may descend under gravity. In such wells there may also be a comparable problem in retrieving the instrument because it may become stuck, or alternatively, the logging cable becomes stuck. This latter circumstance often occurs when the cable, by its longitudinal movement, "key seats" itself in the mud cake layer which covers the surface of permeable rock formations. In fact the cable may slice into the rock itself, being plastered against the borehole wall by hydraulic forces in the borehole. Often the cable will be broken before sufficient pulling force is applied to cause the cable and attached instrument to move upwardly. In descending, the force of gravity is insufficient to overcome these same effects and the instrument may move discontinuously or become stuck.
In descending, the matter of primary concern is that the bottom of the well or other objective point in the borehole be reached expeditiously with minimum loss of time. Economy of time is important because of the extremely high cost of drilling operations involving slant holes. These in general are drilled from large expensive drilling platforms situated offshore or in hostile and remote locations. Normally the drilling operation is interrupted while logging operations are conducted; so the "rig time" expended while logging may be an important factor in the cost of the overall operations.
As many as 10 to 30 wells are drilled from a single platform, each well directed outwardly from a central vertical hole to form a cluster that uniformly taps the objective reservoir over a preselected area. To control these directionally drilled wells and assure that they reach and penetrate the objective zone often requires more and different types of well logs than conventionally drilled wells. Various prior art methods for traversing such deviated boreholes have been devised.
One such prior art method is called "pump down" logging. This method involves the use of logging instruments which are small in diameter such that they will pass through the drill pipes. With the drilling bit removed, drill pipe is inserted into the hole to a depth above the portion of the hole desired to be logged. At this point, the drill pipe is kept more or less stationary while mud fluids are continuously circulated. The drill pipe is slowly reciprocated longitudinally to avoid its becoming stuck in the hole due to key seating or other hydraulic forces like those discussed previously in respect to the logging cable.
A logging instrument, attached to the conventional logging cable is placed in the top end of the drill pipe and is carried to the bottom by the descending column of mud fluids. The logging cable is meanwhile paid out by the cable winch to allow the tool to descend. When the instrument has reached the bottom of the drill pipe and emerges into the "open" hole, it may fall under gravity or it may become stuck just as in normal operations depending on the conditions of the hole and its inclination. If it becomes stuck, the necessary procedure has been to withdraw the instrument from the hole, add more sections of pipe to the drill pipe string and try again. In some cases it has been found impossible to obtain "open hole" logs of lower portions of steeply inclined boreholes because no satisfactory method was available for overcoming these obstacles. In fact, due to the fact that so much time is consumed in "pump down" logging, with its attendant hight cost, the decision is often made to proceed with a minimum number of logs, or even with none at all. This results in the possible consequence that an unproductive well will result.
A more recent prior art method is the placing of a means of motivating power within the logging instrument itself. A command generated at the surface is used to initiate the motive power to apply force for causing the instrument to move within the borehole. Various methods of accomplishing this are known in the art. An example of such art is disclosed in applicant's above-mentioned U.S. Pat. No. 4,109,521. However, even with motive means within the instrument, problems may occur in causing the instrument to traverse the borehole. For example, if the instrument is being lowered into the borehole and becomes stuck, by the time it is realized at the surface that downward tool movement has stopped and the motive means commanded on, the logging cable may have been played out to overrun the tool and may kink or twist so that when the motive means engages, the cable may be pinched or otherwise damaged. Also, when the instrument together with the lower portion of the cable cease to move, their stuck condition quickly begins to worsen due to continued depositions of mud cake wherever mud filtrate can enter permeable formations. This effect causes the cable and instrument to be more forcefully plastered against the borehole wall and progressively more difficult to disengage. Alternatively, as the instrument is being raised within the borehole, it may become stuck and prior to receipt on the surface of an indication of this condition, the cable may be stretched to an extent which may either snap the cable or at least pull it from its connection with the instrument.
Yet another objectional consequence of discontinuous or non uniform movement of the logging instrument relates to the requirement while logging to produce a record of the logging parameters on a linear depth scale. This is accomplished by a means old in the art whereby the recorder is driven by movement of the sheave wheel over which the cable travels. If the sheave movement is not synchronous with the instrument movement, then the logged parameters will be erroneously recorded with reference to the depth scale. This is particularly prevalent when logging downward in a well, and for this reason in the prior art it is exceptional to undertake a logging operation while going into a well with a logging device. In fact it is sometimes preferred to log while moving downward, as for example when excessively high temperatures are to be encountered. In such case, by logging downward it is possible to obtain a log of the greatest possible portion of the well before the instrument fails. In another case, if the instrument contains a neutron source initiated below a gamma ray measuring device, then radioactivity may be induced in formation elements and detected by the gamma ray measuring device only if the instrument is moving downward. Thus it may be desirable in some cases to log in both directions in order to obtain two differing measurements. In yet another case, a complex instrument may comprise so many measuring devices that it is not practical to perform all the measurements simultaneously. In such case a selected group of measurements may be made going into the hole and the remainder may be made on the way out.
The instant invention, by providng correct correlation between measured parameters and depth in the hole makes is feasable to produce logs of acceptable quality while logging in either direction by overcoming the problems of the prior art due to discontinuous and non uniform drag caused by friction, viscous and hydraulic forces, obstacles in the borehole, and the like.
The present invention overcomes the deficiencies of the prior art and novel means and apparatus are disclosed for providing synchronized cable playout to facilitate movement of the logging instrument within the borehole and for the generation of command signals for initiating a motive force contained within the logging instrument to facilitate movement thereof within the borehole.